Dr. Julie, a.k.a. Scientific Chick, brings you insights into what's happening in the world of life sciences. Straight from the scientific source, relevant information you should know about, in plain language.

Monday, July 16, 2012

I think it’s pretty safe for me to assume that you’re
sitting down right now. Some of you might be so lucky to have a treadmill desk
(on my long list of wants) or you might be reading this on your mobile device
while waiting in line at the grocery store, but chances are most of you are
sitting. Many people around the world do what I call “sitting for a living”,
meaning working at a desk for the larger portion of the day. A recent study
reported that Taiwan, Norway, Saudi Arabia and Japan are the “most seated”
countries with people sitting over 6 hours per day. It’s a no-brainer that
sitting all day is bad for your health – it’s been associated with health
issues like Type 2 diabetes and cardiovascular disease. Conversely, it’s also
known that exercise is good for you and helps prevent health issues like – you
guessed it – Type 2 diabetes and cardiovascular disease. Based on these
findings, the government has established guidelines suggesting adults get at
least 150 minutes of moderate to vigorous exercise per week to benefit from
physical activity.

Long work hours combined with all this evidence about the
health benefits of exercise has led to a new label: what I call the “active
couch potato”, one who is both sedentary and physically active. These people,
like me, work 8- to 10-hour days at a desk, but also exercise – for example,
they cycle to work, or jog regularly. So the question becomes: is regular
exercise sufficient to offset sitting all day on most days?

A recent study tries to shed light on the relationship
between sitting and life expectancy, regardless of exercise. After seeing a
number of dubious headlines about this study such as “Sitting less can boost
your life expectancy” and “Sitting ourselves to death” (gasp!), I thought I’d
take a look at the original research and report back.

The researchers carried out a “meta-analysis”, which
essentially means an “analysis of analyses”. They looked through previously
published scientific articles on the topics of sitting and life expectancy,
pooled the results together, and crunched the numbers to get a “big picture”.
To make their results easier to understand, they divided sitting habits in
three levels: level 1 means sitting less than a quarter of the time in a normal
day, level 2 means sitting about half the time, and level three means sitting
three quarters of the time or more. What their results show is that going from
level 2 or 3 to level 1 (meaning sitting less) can mean a gain of 2 years in
life expectancy.

So what? Do we really conclude that sitting is a silent
killer?

Not so fast. We have many reasons to believe that sitting is
bad for our health, but this article isn’t necessarily one of them. For one
thing, the researchers themselves point out that their results don’t mean that
someone who sits less can expect to live longer. Life expectancy is a measure
that can only apply to populations, not to individuals, so while the study is
relevant in a “big picture” kind of way, it means little for your own personal
habits and life expectancy.

The authors also make one big assumption in their model:
that sitting more causes a shortened
life expectancy. However, the study results don’t tell us whether there is a
direct cause-and-effect relationship. I talked about confounders in previous posts, and this is a perfect place to continue this discussion.
For my new readers, confounders are variables that the researcher didn’t take
into consideration, which could explain his results in a different way. For
example, in this instance, the researchers find that sitting more shortens life
expectancy. However, the real explanation could be that people who sit more end
up eating more spicy Cheetos, and it’s the chemicals in the spicy Cheetos that
actually shortens their life expectancy. See what I mean? Of course this is a
silly example, but you get the idea (and of course I will never give up spicy
Cheetos).

Can you think of possible (silly or not!) confounders in
this study? Share in the comments!

You can count on more articles about sitting in the future.
Since this one didn’t really answer whether sitting and exercising cancel out,
I’ll keep looking.

Reference: Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis. (2012) Katzmaryk PT, Lee I-M. BMJ Open 2:e000828.

About Me

Dr. Julie is an Assistant Professor of Neurology at the National Core for Neuroethics and the Djavad Mowafaghian Centre for Brain Health at the University of British Columbia. She holds a PhD in Neuroscience.